How it's made
Polyethylene (PE) is a major commodity plastic. Several types of PE exist and are classified according to their density, ranging from 0.88 to 0.96 g/cm3: linear low-density PE (LLDPE), low-density PE (LDPE), medium-density PE (MDPE), and high-density PE (HDPE).
Like many thermoplastics, PE is a downstream product of the petrochemical industry. However, it may also be derived from the dehydration of ethanol. The starting monomer required for PE synthesis is ethylene, also known as ethene, which is a gaseous hydrocarbon.
Where initial processes relied on extremely high pressures to produce LDPE, later developments involved catalysts that allowed the polymerization reaction to occur under mild conditions, thus enabling the controlled synthesis of a range of PE possessing distinct characteristics. Such innovations include Phillips’ chromium-based catalyst and Ziegler’s improved titanium-based catalytic system; by the 1950s, both systems were utilized in the industrial production of HDPE. Further advancements came with Kaminsky and Sinn’s introduction of metallocene-based catalysis in the late 1970s. These catalysis methods represent the basis of how a range of PE resins are synthesized to this day.
About this material
The properties of PE may vary depending on the process conditions, molecular weight, and molecular weight distribution. Additionally, these properties may be fine-tuned through crosslinking or copolymerization. PE’s primary use remains in packaging, with key examples including plastic bags, bottles, and cups.
Most types, including LDPE, HDPE, and MDPE, exhibit excellent chemical resistance to strong acids and alkalis, as well as mild oxidants and reducing agents. However, they are susceptible to aromatic hydrocarbons or chlorinated solvents. As a highly non-polar material, it repels water and has low moisture permeability, making it suitable for electrical insulation applications. However, it allows for gas exchange, meaning that flavorings, oxygen, and carbon dioxide can pass through it.
Heat
Polyethylene melts in the temperature range of 105–115 °C (221–239 °F) for LDPE and 120–130 °C (248–266 °F) for MDPE, with 144–146 °C being the theoretical limit reported for PE. It demonstrates low thermal conductivity and can, therefore, be used as an insulator in applications where heat transfer needs to be minimized. Additionally, it demonstrates a low dielectric constant, which makes it an effective material for high-frequency insulation needs, as it will minimize energy loss. PE also exhibits a high thermal expansion rate. PE burns slowly; the material will continue to burn even after the removal of the flame source, melting rather than charring, producing a drip.
Grades
LDPE exhibits high degrees of branching in the polymeric structure, meaning the polymer chains do not pack as efficiently into the crystal structure. As a consequence, the material is soft and flexible, exhibiting increased ductility and a lower tensile strength than other types of PE. These characteristics make it suitable for applications such as plastic bags and squeezable bottles. LDPE also exhibits the greatest transparency among PE types.
On the other hand, HDPE (density ≥ 0.94 g/cm3) is more rigid and exhibits a higher tensile strength. By using specific catalysts and reaction conditions, the degree of branching can be controlled and minimized, thus achieving a more efficiently packed crystal structure, which gives HDPE its rigidity and strength, albeit with lower transparency. These characteristics make HDPE ideal for applications like detergent bottles, garbage containers, and water pipes.
LLDPE was synthesized such that it combines flexibility with strength. It demonstrates improved resistance compared to LDPE, including improved stress-cracking and puncture resistance. However, it is more challenging to process. Its relative transparency and strength enable LLDPE’s application for cable coverings and film applications such as Saran wrap or bubble wrap.
For high-performance applications, an extremely tough type of polyethylene was developed, namely Ultra-High Molecular Weight Polyethylene (UHMWPE). Its toughness and excellent wear, impact, and chemical resistance result from its high molecular weight, although this also leads to a less efficiently packed crystal structure and lower densities than HDPE. Notable applications include bearings, gears, artificial joints, and butcher’s chopping boards.
Sustainability
Polyethylene is recyclable, particularly LDPE and HDPE, which can be melted and remolded. However, this process is dependent on proper sorting and handling. PE is non-biodegradable.
History
Polyethylene was first discovered accidentally in the late 1890s by a German chemist exploring diazomethane and later in the 1930s in an industrial setting by British chemists working for Imperial Chemical Industries (ICI). By 1935, a reproducible process of synthesizing PE was established within ICI, with industrial production of LDPE beginning in 1939. In the USA, commercial production began in 1944 at DuPont and Union Carbide Corporation under license from ICI.